1. Field of the Invention
The present invention relates to a process for preparing low molecular weight, highly reactive polyisobutylene in the liquid phase using a boron trifluoride complex catalyst, the polymerization being conducted such that, at the end of the polymerization, the residual isobutene content is less than 2% by weight, the complex catalyst is removed and recycled to the polymerization.
2. Discussion of the Background
Low molecular weight and high molecular weight polyisobutenes having molecular weights of up to several 100,000 Dalton have long been known and their preparation is described, for example, in H. Gxc3xcterbock: Polyisobutylen und Mischpolymerisate, pages 77 to 104, Springer, Berlin 1959. The currently available polyisobutenes of this molecular weight range are mainly prepared with the aid of Lewis acid catalysts, such as aluminum chloride, alkylaluminum chlorides or boron trifluoride, and generally have a molecular weight distribution (polydispersicity [sic]) of from 2 to 7.
A distinction must be made between these conventional polyisobutenes having average molecular weights of from 500 to 5000 Dalton and the highly reactive polyisobutenes, which typically have a high vinylidene group content of preferably substantially more than 60 mol % and a polydispersity {overscore (M)}w/{overscore (M)}n of less than 2. Such highly reactive polyisobutenes are used as intermediates for the preparation of additives for lubricants and fuels, as described for example, in DE-A 27 02 604. For the preparation of these additives, alternating copolymers, in particular polyisobutenylsuccinic anhydrides, are first produced by reacting the terminal double bonds of the polyisobutene with maleic anhydride, and said copolymers are then reacted with certain (poly)amines and/or alcohols to give the finished additive. Since the vinylidene double bonds are preferred reaction sites in the ene reaction with maleic anhydride, whereas, depending on their position in the macromolecule, the double bonds present further in the interior of the macromolecules lead to substantially lower, if any, conversion without the addition of halogens, the amount of terminal double bonds in the molecule is the most important quality criterion for this type of polyisobutene. 
The polyisobutene cation I formed in the course of the polymerization reaction may be converted into the corresponding polyisobutene by elimination of a proton. The proton may be eliminated from one of the xcex2-methyl groups or from the internal xcex3-methylene group. Depending on which of these two positions the proton is eliminated from, a polyisobutene having a vinylidene double bond II or having a trisubstituted double bond III present close to the end of the molecule is formed.
The polyisobutene cation I is relatively unstable and attempts to achieve stability by rearrangement to form more highly substituted cations, if the acidity of the catalyst system is high enough. Both 1,3-methyl group shifts to give the polyisobutene cation IV and successive or concerted 1,2-hydride group and 2,3-methyl group shifts to give the polyisobutene a cation V may take place. Depending on the position from which the proton is eliminated, in each case three different polyisobutene double bond isomers can form from the cations IV and V. However, it is also possible for the cations IV and V to undergo further rearrangement, causing the double bond to migrate further into the interior of the polyisobutene macromolecule.
All these deprotonations and rearrangements are equilibrium reactions and therefore reversible, but in the end the formation of more stable, more highly substituted cations and hence the formation of polyisobutenes having an internal double bond with establishment of the thermodynamic equilibrium is preferred. These deprotonations, protonations and rearrangements are catalyzed by any traces of acid present in the reaction mixture, but in particular by the actual Lewis acid catalyst required for catalyzing the polymerization. Because of these facts and since only polyisobutenes having vinylidene double bonds according to formula II react very well with maleic anhydride with adduct formation, polyisobutenes of the formula III have in comparison substantially lower reactivity and other polyisobutenes having more highly substituted double bonds enter into the ene reaction with maleic anhydride virtually only under isomerizing conditions, the continued efforts of many research groups to find improved processes for the preparation of highly reactive polyisobutenes having higher and higher contents of terminal double bonds is understandable.
The preparation of low molecular weight, highly reactive polyisobutene from isobutene or hydrocarbon streams comprising isobutene, in particular from C4 cuts, substantially free from 1,3-butadiene originally present therein, from steam crackers, FCC crackers (FCC: Fluid Catalyzed Cracking), i.e. C4 raffinates, is known from a number of patents, for example from EP-A 145 235, EP-A 481 297, DE-A 27 02 604, EP-A 628 575, EP-A 322 241 and WO 93/10063. All these processes relate to the polymerization of isobutene in a single polymerization stage.
A further improvement is provided by the two- or multi-stage process of WO 96/40808, which comprises carrying out the polymerization reaction in at least two polymerization stages, the added isobutene at a substantially constant isobutene concentration being polymerized to a partial conversion of up to 95% in the first polymerization stage and the polymerization of the remaining isobutene being continued in one or more subsequent polymerization stages, without or after prior isolation of the polyisobutene formed in the first polymerization stage.
In addition to the efforts to optimize the process as described in the abovementioned publications, it was also attempted to recover BF3 for economic and ecological reasons. Thus, EP-A 0 742 191 suggests the thermal decomposition of the BF3 complex in the product stream and the absorption of the liberated BF3, for re-use, in an olefin stream comprising a promoter.
This process has the disadvantages that the product of value is subjected to thermal stress in the presence of the catalyst which results in isomerization of the vinylidene double bond to give the more highly substituted double bond type, and that the wastewater is polluted by the complexing agent. The suggested method is not practical for the preparation of reactive polyisobutenes.
It is an object of the present invention to provide a process which makes it possible to prepare highly reactive polyisobutylene and to recycle the catalyst.
We have found that this object is achieved by a process for preparing low molecular weight, highly reactive polyisobutylene having an average molecular weight Mn of from 500 to 5000 Dalton and a terminal double bond content of more than 80 mol % by polymerization in the liquid phase of isobutene or hydrocarbon streams comprising isobutene with the aid of a boron trifluoride complex catalyst at from xe2x88x9240 to +20xc2x0 C. and at from 1 to 20 bar, which comprises
a) polymerizing until the residual isobutene content of the reaction mixture is less than 2% by weight, based on the total amount of streams introduced, or removing residual isobutene towards the end of the polymerization until the residual isobutene content is less than 2% by weight,
b) enriching the boron trifluoride complex catalyst which is obtained here in the form of droplets in the disperse and/or coherent phase,
c) recycling the complex-enriched phases to the polymerization and
d) compensating for catalyst losses by adding boron trifluoride and, if necessary, complexing agents.